1. Field of the Invention
The present invention relates generally to a method and apparatus for generating feedback information in a cellular mobile communication system including a plurality of Base Stations (BSs), and more particularly, to a method and apparatus for transmitting and receiving feedback information in a Coordinated Multi-Point (CoMP) system, in which a plurality of BSs cooperate to support downlink transmission to a User Equipment (UE).
2. Description of the Related Art
Mobile communication systems have been developing into high-speed, high-quality wireless packet data communication systems to provide data services and multimedia services beyond traditional voice-oriented services.
Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) proposed by the 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) proposed by the 3GPP2, and 802.16 proposed by the Institute of Electrical and Electronics Engineers (IEEE), have been developed to support high-speed, high-quality wireless packet data transmission services.
The 3G wireless packet data communication systems, such as HSDPA, HSUPA and HRPD, use technologies such as Adaptive Modulation and Coding (AMC) and channel-sensitive scheduling to improve transmission efficiency. In AMC and channel-sensitive scheduling, a transmitter applies a suitable Modulation and Coding Scheme (MCS) at the most efficient time determined based on partial channel state information fed back from a receiver.
With the use of AMC, the transmitter adjusts the amount of transmission data according to the channel state. That is, in a poor channel state, the transmitter reduces the amount of transmission data to decrease a reception error probability to a desired level. In a good channel state, the transmitter increases the amount of transmission data to increase the reception error probability to a desired level, thereby ensuring efficient information transmission.
Additionally, with the use of channel-sensitive scheduling resource management, the transmitter selectively services a user having a superior channel state among several users, contributing to an increase in system capacity, compared to the case in which the transmitter allocates a channel to one user and services the user. Such an increase in system capacity is called ‘multi-user diversity gain’. When AMC is used together with Multiple Input Multiple Output (MIMO), the function of determining the number of spatial layers or the rank of a transmission signal can be adopted. A wireless packet data communication system adopting AMC takes into account the number of layers for MIMO transmission as well as a coding rate and a modulation scheme in determining an optimum data rate.
In general, Orthogonal Frequency Division Multiple Access (OFDMA) is a technology that can increase capacity, as compared to Code Division Multiple Access (CDMA). One of several reasons for increasing capacity in OFDMA is that frequency-domain scheduling is possible.
With the use of channel-sensitive scheduling, a capacity gain is obtained based on a property that a channel changes over time. Likewise, more capacity gain is achieved by utilizing another property that a channel changes in frequency. In this context, replacing CDMA used in 2nd Generation (2G) and 3G mobile communication systems with OFDMA for future-generation systems has recently been studied. 3GPP and 3GPP2 started to work on standardization of an evolved system using OFDMA.
FIG. 1 illustrates a cellular mobile communication system in which a Transmission (Tx)/Reception (Rx) antenna is included at the center in each cell.
Referring to FIG. 1, in a cellular mobile communication system including a plurality of cells, a User Equipment (UE) receives a mobile communication service using the aforementioned techniques from a selected cell during a semi-static time period. In FIG. 1, it is assumed that the cellular mobile communication system includes three cells 100, 110 and 120 (Cell 1, Cell 2 and Cell 3). Cell 1 provides the mobile communication service to UEs 101 and 102 (UE 1 and UE 2), Cell 2 provides the mobile communication service to a UE 111 (UE 3), and Cell 3 provides the mobile communication service to a UE 121 (UE 4). Antennas 130, 131 and 132 are included at the centers of the respective cells 100, 110, and 120. The antennas 130, 131, and 132 correspond to BSs or relays.
UE 2, receiving the mobile communication service from Cell 1, is relatively far from the antenna 130, as compared to UE 1. Moreover, Cell 1 supports a relatively low data rate for UE 2 because UE 2 experiences severe interference from the antenna 132 at the center of Cell 3.
If Cell 1, Cell 2 and Cell 3 provide mobile communication services independently, they transmit Reference Signals (RSs) so that a downlink channel state is measured on a cell basis. In a 3GPP LTE-A system, a UE measures a channel state between the UE and a BS using Channel State Information-Reference Signals (CSI-RSs) and feeds back channel state information to the BS.
FIG. 2 illustrates the positions of CSI-RSs transmitted from BSs to a UE in an LTE-A system.
Referring to FIG. 2, resources available in the LTE-A system are divided into equal-size Resource Blocks (RBs). The horizontal axis and vertical axis of the resources represent time and frequency, respectively. Signals for two CSI-RS antenna ports are transmitted in the resources of each of RBs 200 to 219. That is, the BS transmits two CSI-RSs for downlink measurement to the UE in the resources of the RBs 200.
In a cellular mobile communication system including a plurality of cells as illustrated in FIG. 1, an RB at a different position is allocated to each cell and CSI-RSs are transmitted in the resources of the allocated RB. For example, in FIG. 1, Cell 1 transmits CSI-RSs in the resources of the RBs 200, Cell 2 transmits CSI-RSs in the resources of the RBs 205, and Cell 3 transmits CSI-RSs in the resources of the RBs 210. The reason for allocating different RBs (e.g., different time and frequency resources) for CSI-RS transmission to different cells is to prevent mutual interference between CSI-RSs from different cells.
A UE estimates a downlink channel using CSI-RSs, generates a Rank Indicator (RI), a Channel Quality Indicator (CQI), and a Precoding Matrix Index (PMI) as CSI of the estimated downlink channel, and feeds back the CSI to a BS. There are four modes defined for periodic CSI feedback on a Physical Uplink Control Channel (PUSCH) from a UE.
1. Mode 1-0: RI, wideband CQI (wCQI)
2. Mode 1-1: RI, wCQI, wideband PMI (wPMI)
3. Mode 2-0: RI, wCQI, subband CQI (sCQI)
4. Mode 2-1: RI, wCQI, wPMI, sCQI, sPMI
The feedback timing of each piece of information in the four feedback modes is determined according to Npd, NOFFSET,CQI, MRI, and NOFFSET,RI indicated by higher-layer signaling. In Mode 1-0, the transmission period of a wCQI is Npd and the feedback timing of the wCQI is determined using a subframe offset of NOFFSET,CQI. Additionally, the transmission period and offset of an RI are Npd·MRI and NOFFSET,CQI+NOFFSET,RI, respectively. Mode 1-1 and Mode 1-0 have the same feedback timing, however, a PMI is transmitted together with a wCQI at the transmitting timing of the wCQI in Mode 1-1. FIG. 3 illustrates the feedback timings of an RI, a wCQI, and a PMI in Mode 1-0 and Mode 1-1. Each transmission timing is represented as a subframe index.
In Mode 2-0, the feedback period and offset of an sCQI are Npd and NOFFSET,CQI, respectively. The feedback period and offset of a wCQI are H·Npd and NOFFSET,CQI, respectively. Herein, H=J·K+1, where K is a value indicated by higher-layer signaling and J is a value determined by a system bandwidth. For instance, J is 3 for a 10-MHz system. Thus a wCQI is transmitted, substituting for a sCQI at every H sCQI transmissions. The feedback period and offset of an RI are MRI·H·Npd and NOFFSET,CQI+NOFFSET,RI respectively. Mode 2-1 is the same as Mode 2-0 in feedback timing but different from Mode 2-0 in that a PMI is transmitted together with a wCQI at the transmission timing of the wCQI. FIG. 4 illustrates the transmission timings of an RI, an sCQI, a wCQI, and a PMI in Mode 2-0 and Mode 2-1 under the condition that Npd=2, MRI=2, J=3 (10 MHz), K=1, NOFFSET,CQI=1, and NOFFSET,RI=−1.
The above-described feedback timings are set for 4 or fewer CSI-RS antenna ports. For 8 CSI-RS antenna ports, two PMIs are fed back, unlike the above cases. For 8 CSI-RS antenna ports, Mode 1-1 is further divided into two submodes. A first PMI is transmitted together with an RI and a second PMI is transmitted together with a wCQI in a first submode. The feedback period and offset of the RI and the first PMI are defined as MRI·Npd and NOFFSET,CQI+NOFFSET,RI, respectively and the feedback period and offset of the wCQI and the second PMI are defined as Npd and NOFFSET,CQI, respectively.
For 8 CSI-RS antenna ports, a Precoding Type Indicator (PTI) is added in Mode 2-1. The PTI is transmitted together with an RI in a period of MRI·H·Npd with an offset Of NOFFSET,CQI+NOFFSET,RI. If the PTI is 0, first and second PMIs and a wCQI are feedback. The wCQI and the second PMI are transmitted at the same timing in a period of Npd with an offset of NOFFSET,CQI. The feedback period and offset of the first PMI are H·Npd and NOFFSET,CQI, respectively. H′ is indicated by higher-layer signaling. On the other hand, if the PTI is 1, the PRI and the RI are transmitted together and the wCQI and the second PMI are transmitted together. The sCQI is additionally fed back. The first PMI is not transmitted. The PTI and the RI have the same feedback period and offset as those of the PTI and RI in the case in which the PTI is 0. The feedback period and offset of the sCQI are defined as Npd and NOFFSET,CQI, respectively. The wCQI and the second PMI are fed back in a period of H·Npd with an offset of NOFFSET,CQI. H is the same as that for 4 CSI-RS antenna ports. FIGS. 5 and 6 illustrate transmission timings when PTI=0 and PTI=1 in Mode 2-1 for 8 CSI-RS antenna ports under the condition that Npd=2, MRI=2, J=3 (10 MHz), K=1, H′=3, NOFFSET,CQI=1, and NOFFSET,RI=−1.
The conventional CSI feedback technology is based on the premise that a UE transmits a single CSI feedback, without regard to a multi-CSI feedback situation for CoMP transmission, that is, simultaneous transmissions from a plurality of transmission points.